1. Field of the Invention
The present invention relates to processes for producing coke, an artificial graphite having a high graphitization degree, a carbon material for a negative electrode of non-aqueous solvent type secondary battery having a high discharge capacity and a high charge-discharge efficiency, from mesophase pitch (optically anisotropic pitch), and a pitch composition mainly used for the production of these substances.
2. Description of the Prior Art
Mesophase pitch is an excellent carbon material capable of producing pitch coke having a high graphitizability at a high yield. However, when the mesophase pitch is held in an electric furnace and heat-treated therein, gases are generated therefrom, so that the pitch is foamed and the volume thereof increases several tens times. Thus, the production of coke from the mesophase pitch has a problem concerning its productivity. Also, in the case where the mesophase pitch is used as a binder to produce a carbon-based molding material, the pitch is foamed by gases generated. As a result, there arises such a problem that coke derived from the mesophase pitch which is contained in the carbon-based molding material exhibits a low density.
Japanese Patent Application Laid-open No. Heisei 6(1994)-299076 discloses a method of adding carbon black to mesophase pitch in order to inhibit the foaming of the mesophase pitch.
However, the coke produced by the above method of adding carbon black to mesophase pitch, shows a poor graphitizability due to strong interaction between carbon black and mesophase pitch, thereby failing to obtain an artificial graphite having a high graphitization degree. Therefore, it has been required to provide not only a process for producing high-density coke at a high productivity while avoiding foaming of the mesophase pitch, but also a process for producing an artificial graphite having a high graphitization degree.
In addition, recently, lithium ion secondary batteries having a negative electrode made of a carbon material, have been rapidly put into practice as a power source for various electronic devices used in the current high information-oriented society, because these batteries show a high voltage and a high-energy density and are excellent in safety and cycle characteristics.
Natural graphite conventionally used exhibits a high discharge capacity due to its higher crystallinity as compared to those of other carbon materials. However, it is required that the natural graphite is pulverized in order to prepare a negative electrode material therefrom. Therefore, the pulverized natural graphite has a large surface area, which results in low charge-discharge efficiency at initial cycle. In addition, the natural graphite is deteriorated in cycle life since it contains a large amount of impurities such as metal components.
Accordingly, it has also been required to provide carbon materials containing a less amount of impurities such as metal components and exhibiting a high charge-discharge efficiency at initial cycle. As carbon materials satisfying such a requirement, Japanese Patent Application Laid-open No. Heisei 10(1998)-121054 discloses a graphite powder containing a less amount of impurities and exhibiting a crystallinity compatible to that of natural graphite, which is produced by heat-treating specific mesophase pitch in a specific temperature range in a non-oxidative atmosphere, and then successively subjecting the heat-treated material to pulverization and graphitization.
However, the graphite powder obtained in Japanese Patent Application Laid-open No. Heisei 10(1998)-121054 has a highly-oriented flow structure. Therefore, when such a graphite powder is used as an electrode material for secondary batteries, there is caused such a problem that the solvent contained in an electrolyte solution shows a high decomposition activity upon charging due to the crystal structure on the surface of the graphite powder, which results in deteriorated charge-discharge efficiency of the batteries. Further, as described above, the mesophase pitch is undesirably foamed by gases generated when heat-treated in an electric furnace, so that the volume thereof increases up to several tens times, thereby causing the problem concerning its productivity. Accordingly, it has been demanded to provide a process for producing a graphite powder capable of realizing not only a high productivity but also a high discharge capacity and a high charge-discharge efficiency of finally produced batteries.